Double band presses may be used for the continuous production of endless web-shaped work materials, particularly for the production of decorative laminated materials, copper-lined electrolaminates, thermoplastic webs, chipboards, fiberboards and the like. These double band presses may comprise two continuous circumferentially extending press bands between which the material web is cured under pressure and possibly heated while being transported in the feed direction. Pressure plates in the press frame of the double band press produce the area pressure transmitted from the pressure plates to the inner sides of the press bands and from the press bands to the work material web.
Two different working principles for transmitting the pressure from the pressure plate to the inner side of the press band involve the isobaric and isochoric machines. In isobaric machines (see, e.g. DE-PS 27 22 197), annularly self-contained sliding-surface or packing or floating seals are arranged along an edge of the pressure plate and placed against the press band so that the press band slides during the operation of the double band press. Pressure chambers are thereby formed which are defined, in the vertical direction, by the pressure plate and by the inner side of the press band and, in the horizontal direction, by the floating seals. A fluid pressure medium, e.g. oil or compressed air, is introduced into these pressure plates.
In isochoric machines, the pressure is mechanically transmitted via rolls, from the pressure plate to the inner side of the press band. A stationary roll bed arranged in the pressure plate is known from DE-OS 31 23 291. The roll bed comprises rolls which are offset relative to one another and supported by shafts in bearing strips which are fastened in the pressure plate.
DE-OS 33 04 754 discloses a double band press which combines the isobaric and isochoric principles. A roll bed is arranged in a pressure cushion which is sealed at the sides by floating seals. This press can be operated in a purely isochoric manner in that only the roll bed is placed against the press band without introducing a fluid pressure medium under pressure into the pressure cushion. A purely isobaric operation is also possible in that the roll bed is not placed against the press band and only a fluid pressure medium maintains the pressure in the pressure cushion. In the combined isobaric/isochoric type of operation, the roll bed is placed against the press band and a fluid pressure medium is introduced into the pressure cushion. In this double band press, an additional pressure chamber is used for exerting a pressure on the roll bed. This pressure chamber is arranged at the side of the pressure plate remote of the press band and is defined in the vertical direction by the pressure plate and by a closure located in the press frame and in the horizontal direction by lateral seals which are constructed identically to the floating seals for the pressure cushion. The pressure plate and the roll bed located in the pressure plate are placed against the press band by introducing a fluid pressure medium into this pressure chamber.
Unfortunately, however, when such double band presses are operated in the combined isobaric/isochoric operation, a leakage of fluid pressure medium can occur at the lateral seals of the pressure chamber at the side of the pressure plate remote of the press band, particularly at higher pressures. Furthermore, the press frame and material to be pressed can be soiled and often requires treatment when pressing at increased temperature. The pressure plate is heated to the required temperature for this purpose.
Sealing materials for lateral seals comprise plastic or elastomers which can resist temperatures up to only about 270.degree. C. The lateral seals of this pressure chamber are therefore destroyed at higher temperatures, leading ultimately to a failure of the pressure chamber to apply pressure and to a time-consuming and costly repair of the double band press. Such double band presses are consequently not suitable for use at higher temperatures.
To overcome these and other drawbacks of previously known presses, it is an object of the invention to develop an isochoric or combined isochoric/isobaric double band press to operate reliably at increased temperatures.
Another object of the invention is to provide a press in which no leakage of pressure medium can occur in the shape compensating plate due to a hermetic lateral edge seal.
A further object of the invention is to provide a press in which soiling of the press and material to be pressed is reliably prevented.
Another object of the invention is to provide a double band press with a shape compensating plate in which there is much less waste of the material to be pressed than in previous double band presses.
Yet another object of the invention is to provide a double band press in which the shape compensating plate contains no plastic, so that the double band press can also be used for high temperatures.
Still another object of the invention is to provide a double band press which compensates for bending in the pressing region of the press, particularly bending of the press frame by a shape compensating plate so that a product with improved dimensional stability is produced on the double band press.
A still further object of the invention is to provide a press having a good thermal fluid pressure medium in a shape compensating plate, using, for example, metallic fluids which demonstrates good thermal conductivity so that an unimpeded transfer of heat is ensured between the press bands and a heat exchanging plate in a plane-parallel manner in the press frame.
Yet another object of the invention is to provide a double band press in which material of different widths to be processed by using a shape compensating plate divided into a plurality of individual strip-shaped shape compensating plates arranged adjacent to one another.
Additionally, it is an object of the invention to provide a double band press comprising a shape compensating plate which can be used not only in continuously operating double band presses, but also in discontinuously operating single or multiple-platen presses.